In geometry, a **torus** is a solid of revolution generated by revolving a circle about an axis coplanar with the circle. The sphere is a special case of the torus obtained when the axis of rotation is a diameter of the circle. If the axis of rotation does not intersect the circle, the torus has a hole in the middle and resembles a ring doughnut, a hula hoop or an inflated tyre (U.S. tire). The other case, when the axis of rotation is a chord of the circle, produces a sort of squashed sphere resembling a round cushion. *Torus* was the Latin word for a cushion of this shape.

According to the broadest definition, the generator of a torus need not be a circle but could also be an ellipse or any other conic section.

picture of torus

with one homology class

In nuclear physics a **torus** is a large fusion reactor which is very roughly the shape of an elliptical torus. Examples are JET in the UK, JT-60 in Japan, TFTR in the USA and the proposed ITER.